Static friction, so-called stiction, occurs everywhere in the environment. As a rule, the friction when passing from rest to motion (static friction) is greater than the friction during the continuance of the motion (sliding friction). To move an object from its resting position, a force greater than the sliding friction must thus be applied to the object.
The existence of static friction is a general problem in all types of controlling that means that an actuating means is to be stopped and later to be moved again. The term actuating means signifies in this context the subject of the control, i.e. the component/components actuated during the control.
In valves there is static friction, for instance, between the valve spindle and the packing box, especially if this is tightened firmly. Moreover, static friction may occur in other positions in a valve, for instance, between ball and seat in a ball valve.
Static friction manifests itself by so-called stick-slip motion, i.e. the valve sticks owing to the friction in a certain position, which requires a certain amount of force to overcome the resistance from the stiction. Once the static friction resistance has been overcome and the actuating means moves towards its desired position, the applied force is too great relative to the sliding friction, which means that the actuating means will be accelerated and therefore pass the desired position before the control system has time to brake the actuating means. This problem is particularly pronounced when small valve movements are desired, or when the time constant of the control system is great and, consequently, the controlling occurs slowly relative to the motion of the actuating means.
When controlling, for example, a flow passing through a valve, static friction in the valve thus gives rise to oscillations in the controlled flow round the set value thereof. If the static friction increases during operation, for instance, owing to wear or clogging, the size of the oscillations will increase.
It is desirable to reduce the effect of the static friction in valves, of which a large number may be included in process equipment, since the above-described oscillations in processes give rise to an increased power consumption and waste of raw material. As a rule, it is however not economically defensible to interrupt the process and take care of the problems of friction each time an unacceptably great stick-slip motion is discovered in one of the valves. For this reason, it is desirable to be able to compensate for static friction during operation.
A difficulty in compensating for static friction is that this may vary with valve position, time and between different cases of operation. For instance, the valve is in most cases worn unevenly, and therefore the friction is not the same in different positions of the valve. Moreover, variations in temperature cause corresponding variations in static friction, since at high temperatures the material expands and causes increased friction. In a process, the temperature can, of course, vary both in time and between different cases of operation. Also the valve becoming dirty may give rise to variations in friction.
A previously well-known technique of compensating for static friction is called dithering. This technique is disclosed in, inter alia, U.S. Pat. No. 3,562,620 in connection with the controlling of electric motors, and in CH-600,219 in connection with the controlling of a hydraulic valve. Dithering implies that a high-frequency signal is superposed on, i.e. added to, the control signal. The amplitude of the signal should be sufficiently high to overcome the static friction, and the frequency of the signal should be so high that the disturbance generated by such superposing is above the relevant frequency range of the system. The mean value of the superposed signal is zero, which results in the high-frequency valve constantly oscillating on the spot. This high-frequency oscillation prevents the valve from sticking in the above-described fashion, thereby avoiding the problem of stick-slip motion.
The main drawback of dithering is precisely that the valve is forced to oscillate continuously, which, of course, results in an increased wear of all moving parts of the valve. Besides, this technique is not usable to overcome and compensate for static friction in valves where the control signal will be low-pass filtered (integrated) in the actuator, such as in pneumatically controlled valves. This problem will be further discussed below with reference to the drawings.
A further compensating technique, so-called impulsive control, is described in "A Survey of Models, Analysis Tools and Compensation Methods for Control of Machines with Friction", Armstrong-Helouvry et al, Automatica, Vol. 30, No. 7, pp 1083-1138. Impulsive control means that the actual control signal is generated as a sequence of pulses, and therefore both controlling and overcoming of friction are accomplished by means of the same signal. Each pulse results in a certain movement of the actuating means. Variations with respect to time in the static friction consequently cause corresponding variations with respect to time in the movement of the actuating means. To obtain a control signal which as much as possible is independent of any variations in static friction, the pulses must have a high amplitude and a short duration.
Impulsive control suffers from, inter alia, the drawback that the static friction resistance must be known, at least in the sense that the generated pulses must have a sufficient amplitude to overcome every conceivable friction resistance. Moreover, the pulses have a short duration, which means that a high-frequency signal is to be transferred to the actuating means. Therefore, this technique is not applicable in valves having a low-pass filtration of the control signal, for instance, pneumatically controlled valves.